Gene Therapy is an innovative form of medicine. Among several gene delivery vectors currently used in gene therapy trials, naked DNA alone or in combination with others such as liposome, electroporation, or gene gun is employed in almost 25% of approved clinical protocols. Naked DNA, when used alone, is probably the safest and the most convenient form of gene delivery vector. However, its approach has been limited because of its low level of gene expression.
In addition, one of major limiting factors for its large scale clinical application is the low expression level of a therapeutic protein at a given amount of DNA injected to a target site. In previous gene therapy trials, 4 mg of DNA in total was applied to a patient, 2 mg each with an interval of 4 weeks. This is a relatively large amount of DNA and as such, it may contribute to high production costs. The best way of making naked DNA gene therapy affordable is to use an expression system that not only drive the highest possible level of therapeutic protein and but also yield the highest possible copy number in E. coli. 
The present inventors have therefore endeavored to meet the above need, and developed a highly efficient eukaryotic expression vector containing an exogenous transcription regulatory element which comprises a promoter/enhancer and the nucleotide sequence upstream of the translation initiation codon derived from HCMV IE gene or human EF1α gene.